Photopolymerizable epoxy compositions containing epoxy resins and various photoinitiators are well known. However, because the existing systems all suffer from one or more drawbacks, there is a continuing need in the industry for improved photopolymerizable epoxy compositions. Thus, in U.S. Pat. No. 3,074,869, there are disclosed photosensitive epoxy compositions containing a nitrosoamine as a photoinitiator. Compositions of this type require relatively long exposure to a high intensity light source to produce complete polymerization.
In U.S. Pat. Nos. 3,205,157 and 3,708,296, there are disclosed photosensitive epoxy compositions containing aryldiazonium salts of halogen-containing complex anions. Such compositions have limited usefulness because they have poor thermal stability, because their spectral response is limited to the ultraviolet region of the spectrum, and because nitrogen is evolved during photopolymerization causing pinholes and bubbles in heavy coatings of the composition.
When these known aryldiazonium salts are used to induce polymerization of oxetanes, or mixtures of oxetanes with epoxy resins, e.g., as described in U.S. Pat. No. 3,835,003, the same types of problems are encountered. Although several patents describe various techniques for stabilizing mixtures of diazonium salts and epoxides, such techniques are not satisfactory for several reasons. For example, the increase in stability which is obtained is measured only in days. Also, the addition of stabilizers contaminates the compositions with nonreactive material which softens the resulting product and also reduces the rate of photocure. See, e.g., U.S. Pat. Nos. 3,711,390; 3,711,931; 3,816,278; 3,816,280; 3,816,281; and 3,817,845.
Various types of aromatic halonium (typically iodonium or sulfonium) salts have been proposed for use as photoinitiators in photopolymerizable epoxy compositions which do not have the drawbacks associated with nitrosoamine and aromatic diazonium salts.
For example, photopolymerizable epoxy compositions which comprise an epoxy-containing material and a photosensitive aromatic iodonium salt are described in U.S. Pat. No. 4,378,277. The aromatic groups of the iodonium salt may be substituted by one or more organic groups including alkoxy groups (e.g., methoxy, ethoxy, butoxy, and the like).
Reference to radiation curable compositions of an epoxy resin and an aromatic halonium salt is made in U.S. Pat. No. 3,968,056 and further discussed in U.S. Pat. No. 4,026,705 where it is disclosed that the aromatic group of the aromatic halonium salt can be substituted with from 1 to 4 monovalent radicals selected from groups including C.sub.1 to C.sub.8 alkoxy groups.
U.S. Pat. No. 4,101,513 discloses polymerizable compositions of a silane and an aromatic onium salt wherein the silane can be a hydrolyzable epoxy-terminated silane and the onium salt can be a diphenyliodonium salt.
U.S. Pat. No. 4,279,717 discloses epoxy functional diorganosiloxane fluids combined with bisaryl iodonium salts, particularly linear alkylate bisdodecylphenyl iodonium salts, that will rapidly dissolve in polysiloxane base polymer fluids.
U.S. Pat. No. 4,310,469 discloses as photoinitiators the diaryl iodonium salts of the formula: ##STR1## where: M is a metal or metalloid;
Q is a halogen radical; PA1 R.sup.11 is a C.sub.(4-20) radical selected from alkyl, haloalkyl and branched alkyl; PA1 R.sup.12 is selected from C.sub.(13) alkyl, C.sub.(18) alkoxy, nitro and halo; PA1 u is an integer having a value of from 1-4, inclusive; PA1 v is a whole number having a value of 0-3, inclusive; and PA1 w is an integer having a value of from 4-6, inclusive, and the sum of u and v has a value of from 1-4, inclusive.
Whereas the foregoing disclosed aromatic iodonium salts are all adequate for their intended use, improvements in the solubilities of aryl iodonium salts in photopolymerizable epoxy compositions, especially epoxypolysiloxanes, is continually sought by the industry. Increased solubility of the particular photosensitive compound in the photopolymerizable polymer solution will, of course, enhance photocuring of the polymer. This improves the performance of the photopolymerized polymer, such as, for example, when it is employed as a release coating for a pressure sensitive adhesive.
It was against this background that an improved photopolymerizable epoxy composition was sought.